The present invention is directed to molded plastic bottles having a champagne style bottom structure closing the container lower end. The phrase champagne style is in reference to a base having an outside surface rotationally symmetric about a longitudinal axis of the bottle including a convex heel having an upper margin integrally formed with the lower end portion of the bottle sidewall, and a central concavity separated from the convex heel by a continuous standing ring that supports the bottle on any underlying surface.
There has be an increasing demand for a plastic bottle that will satisfy the market demands for packaging beer. Many technical problems associated with packaging beer in plastic containers have been previously addressed. The need for low oxygen transporation has yielded special blended polymers, and the additional of barrier coatings and layers of a variety of materials. Beer is generally carbonated to a level comparable to carbonated soft drinks, so the pressure that any container for beer can be expected to experience is significant. This internal pressure provided by the carbonation proves to be of little consequence for bottles designed for soft drinks that employ a bottom containing a plurality of individual feet. However, such a footed structure has generally been found to be commercially unacceptable as a package for beer to the purchasing public, which has come to expect a champagne style base on beer bottles based on its past experience with glass bottles. The standard champagne base has long been employed with glass bottles to distribute forces exerted on the base due to any internal pressure to the sidewall of the bottle. The standard champagne base shape has evolved in various ways in attempts to better withstand these forces. While the use of a champagne base has proven to be desirable in glass, the application of such designs to plastic containers has proven to be difficult as the strengths and weakness of glass and plastic simply are not the same.
The plastic container industry has found the standard champagne base to be an unacceptable configuration for blow molding of plastic bottles because, for example, the standard champagne dome or push-up has been found to be susceptible to inversion when constructed from plastic. To prevent such inversion, it is common practice to increase the thickness of a majority of the base relative to the thickness of the remainder of the bottle. To achieve this thickness variation, some parisons have been designed to place material concentrations in specific predetermined areas of the base, such as a reinforced hoop in the area of the chime, to increase the bottle resistance to stress. Some champagne base configurations required the use of a parison having a stepped or otherwise specially shaped lower end portion to increase the thickness of the base in a selected area to a thickness that is substantially thicker than in other comparable bases. These configurations represent very difficult manufacturing problems as the parisons require very long reheating schedules to achieve the uniform heating required to permit the biaxial stretching of the parison during formation of the bottle. The very long reheating schedule translates either into a slow manufacturing process or into extraordinary capital outlay for very long reheating paths with lots of sensors and controls for coordinating the parison reheating. Even when the proper reheating schedules are followed, the correct placement of the thicker areas at the desired locations in the bottle does not always occur, which results in bottles that do not remain perpendicular or even fail when placed under pressure due to the internal carbonation.
What is needed is a design for a plastic bottle having a champagne style base that is capable of being blow molded from a parison having a substantially uniform wall thickness that allows for a more straight forward reheating cycle prior to blow molding the container, yet results is a container having a very stable base under conditions that are typically experienced by bottles of beer.
A molded plastic bottle of the present invention has a sidewall and a bottom structure closing the container at a lower end portion of the sidewall that is within the champagne base family. That is, the bottom structure has an outside surface rotationally symmetric about a longitudinal axis of the bottle that includes a convex heel and a central concavity connected together by a seating ring. The convex heel has an upper margin of diameter D integrally formed with the lower end portion of the sidewall of the container. A lower margin of the convex heel defines an outer portion of the standing ring that supports the bottle on any underlying surface. Preferably, the standing ring outer portion has a vertical inside radius of curvature of at least about 0.04 D. The central concavity includes a first surface having a lower most section defining an inner portion of the standing ring. The first surface has a vertical curvature radius of at least about 0.8 D, with the center of curvature being located either inside or outside the first surface. Thus the first surface can be slightly concave or slightly convex, or even conical. The standing ring inner portion and outer portion intersect in an abrupt edge defining the lowermost portion of the bottle, which forms a continuous circle lying in a plane normal to the vertical or longitudinal axis of the container. The inner portion of the standing ring is inclined with respect to the plane containing the abrupt edge of the standing ring at an angle of between 20xc2x0 and 60xc2x0.
The angle of the inner surface defining the standing ring, coupled with the small initial outward radius of the outer surface defining the standing ring, permits sufficient polymer to be blown into the standing ring area to achieve the desired performance characteristics for the bottle without requiring that the parison have a thicken region related to this portion of the bottle as was common in the prior art. Additionally, the abrupt edge defining the lowermost portion of the bottle at the standing ring provides an exceedingly strong resistance to roll-out that appears to minimize the opportunity for bottom failure by way of either inversion or blowout. The inner surface of the abrupt edge is the first surface of the central concavity, which in the preferred embodiments is inclined with respect to the plane of the standing ring at an angle of between about 30xc2x0 and 50xc2x0, and most preferably at an angle of about 40xc2x0. The outer surface of the abrupt edge is formed by the lower margin of the convex heel and preferably has a vertical inside radius of curvature that is between about 0.045 D and 0.095 D.
In a preferred embodiment of the molded plastic bottle of the present invention, the upper margin of the convex heel that merges with the sidewall has a vertical inside radius of curvature of between about 0.7 D and 0.8 D. The upper margin of the convex heel forming the junction with the sidewall is preferably situated at between about 0.35 D and 0.40 D above said plane containing the standing ring. The combined curves of the convex heel portion cause the standing ring circle to have a diameter of between about 0.7 D and 0.8 D which provides the necessary stability for the bottle, while retaining a comfortable holding shape of the bottle.
While the central concavity of a bottle of the present invention might be constructed with only a single internal surface, in the preferred embodiments the central concavity includes a second surface having an outer margin spaced uniformly inward from the standing ring and integrally formed with the first surface. The outer margin of the second surface is generally positioned between about 0.1 D to 0.3 D from the longitudinal axis of the bottle. Preferably, the second surface is downwardly convex and includes a lowermost point coincident with the longitudinal axis of the bottle that is spaced upward from the standing ring plane. Generally, the space between the lowermost point of the second surface and the standing ring plane is between about 0.05 D and 0.3 D. In a preferred embodiment, the second surface has a radius of curvature of between about 0.25 D and 1.3 D.
Bottles of the present invention are observed to maintain the structural integrity of the heel and exhibit minimal roll out thus promoting stability of each bottle both during initial filling and during any extended shelf life. Further, these characteristics are reproducible in the bottle of the present invention at commercially acceptable speeds from a parison of substantially uniform wall thickness. These and other features and advantages of the present invention will be apparent from the following discussion of preferred embodiments of the present invention, which makes reference to the attached drawings exemplifying the best mode of carrying out the present invention as now perceived by the inventors.